Every once in a while I come across something on my computer
that thrills me so much I want to sing.
But I decided to write this web page instead. If you ever heard me sing
you'd be doubly glad I did.

So
why the enthusiasm? Digital cameras, ultrawide and long telephoto lenses
frequently suffer from color
fringing, which appears as magenta and green bands
at contrast boundaries. Color fringing increases with the distance from
the center of the image; it's worst at the corners. It's plainly visible
in the image on the right, which is a highly magnified portion of the left
side of the image below, indicated by the arrow. The image is an ice-coated
tree on the Homestead trail south of Boulder, Colorado, taken with the
Canon FD 20mm f/2.8 lens.

I'm excited because my image editing program,
Picture
Window Pro, has a new feature that allows me to easily eliminate
color fringing. There was nothing I could do about it in the traditional
darkroom.

Color fringing is caused by lateral
chromatic aberration, one of the two types of chromatic
aberration in lenses, both of which arise from dispersion in glass:
the variation in its ability to bend light (its index of refraction) with
wavelength (color). Longitudinal chromatic aberration is the change of
the lens's point of focus for different wavelengths. Lateral chromatic
aberration is the change in a lens's magnification for different wavelengths,
resulting in color fringing. It tends to be worst in highly asymmetric
lenses, particularly in retrofocus ultrawides (<24mm in the 35mm format),
where the rear element may be further from the film than the focal length,
and in true telephotos (>200mm in the 35mm format), which may be shorter
than the focal length. The two types of chromatic aberration are described
in Quality
Criteria of Lenses by Schneider Optics. Paul
vanWalree has an excellent in-depth
discussion. His terminology is a little different: he uses transverse
chromatic aberration (TCA) instead of lateral.

To eliminate color fringing I use Picture
Window Pro's Chromatic Aberration...
transformation, accessed by clicking Transformation,
Color.
This displays the screen shown below. The input image is on the left; the
preview on the right; the Chromatic Aberration correction control box is
below. For the sake of illustration the magnifications are different from
what I normally work with. Both images are enlarged 2:1, meaning two screen
pixels represent one image pixel. Assuming a typical screen resolution
of 72 pixels per inch, 36 image pixels per inch are displayed. The original
image is 3306x2238 pixels. So these
images would be portions of the original image enlarged to 92x62
inches (2.3x1.6 meters). Much
bigger than the 13x19 inches my Epson
1270 can print.

I normally leave the input image at its normal size (so the entire image
is displayed on the screen). A crosshair (a "+" sign) in its center denotes
the center of the transformation. If you cropped the image asymmetrically,
you should move it the center of the original uncropped image. Now I am
ready to eliminate color fringing. I move the sliders, first the Red Shift,
then the Blue Shift until it is minimized. The Red Shift is much more prominent
than the blue, which can be quite subtle. I typically pan the preview image
(on the right) to the four corners of the image to make sure the transformation
is centered properly. When I'm satisfied, I click OK.
The settings for red and blue shift should be fairly consistent for a given
lens. But it's so easy to see the correction in the preview window-- it
appears about a second after moving the slider-- that there's no need to
remember it.

After you remove the chromatic aberration there will be colored bands
at the margins of the image. You will need to crop these out. Click on
Transformation,
Geometry,
Crop/Add
border..., then remove one or two pixels from the margins. Set the
preview image to 1:1 and scroll to the corners to make sure you've removed
the right amount. Then click
OK.

You should correct chromatic aberration before
sharpening. I don't know the fine details of sharpening algorithm, but
it is likely performed in HSV or HSL color space on the V (Value) or L
(Lightness) boundaries, which will be much more distinct after removing
chromatic aberration. As I indicated in Understanding
image sharpness and MTF curves, Part 2, sharpening is an integral part
of the digital imaging process, quite necessary for maximizing quality.
Unsharp mask is the preferred technique because it allows you to set a
threshold so you can avoid exaggerating the grain in smooth areas such
as skies. The effects of unsharp masking on a portion of the central tree,
about 2/3 of the way up from the bottom, are shown below. Before I click
OK, I pan the Preview image to make sure I'm not oversharpening (I had
to reduce Amount from 100%) and Threshold isn't set too high (which reduces
the effective sharpening) or too low (which allows skies to become grainy).

The final image, on the right, is strikingly sharp. It's actually slightly
oversharpened, but you won't notice it in the final print. It's evident
that the original negative is extremely sharp, with detail beyond the capacity
of the 2400 dpi scanner to resolve. The Canon FD 20mm f/2.8 is "an excellent
lens," as defined inUnderstanding image
sharpness and MTF curves, especially after chromatic aberration is
removed. Now you know why I'm writing this. The artist and gadget
lover in me is trying to justify the purchase of a new Nikon 4000 dpi scanner
tho the cheapskate in me.

Flo's Undistort Filter
is a Photoshop plugin that performs a CA-removal similar function. It also
corrects distortion.

If I were to sing,
I'd start with "Nessun Dorma," Puccini's heart-wrenching ode to insomnia
from his opera "Turandot." Most appropriate since the muse that drives
me to write these pages often wakes me up in the middle of the night. This
magnificent song expresses the hero's romantic dilemma. He's in love, but
the object of his affection is a serial killer, and he's next on her list.
She allows nobody to sleep, on pain of death, until she learns his name,
which will give her the privelege of dismembering him. What a sweetie-pie!
But all will be forgiven because she's a princess and she's beautiful.
Bill Clinton won't be so fortunate. You gotta love opera.